In their experiments, Hasan and colleagues and Ding and colleagues used
angle-resolved photoemission spectroscopy (ARPES) to detect the Fermi
arcs, characteristic of Weyl nodes, on the surface of TaAs. ARPES is an
ideal tool for such a purpose. The technique involves shining light on a
surface and measuring the energy and momentum of ejected electrons.
This allows for the explicit determination of both bulk nodes and the
Fermi-arc surface states. Ding’s team used an interesting strategy to
identify a Fermi arc and distinguish it from a more conventional closed
Fermi surface (Fig. 1).
They defined a closed contour in the momentum space spanned by their
measurements and investigated how many times surface states at the Fermi
energy crossed this contour. Such a contour will intersect a regular
Fermi surface an even number of times. But it will intersect a Fermi arc
an odd number of times if the arc encloses the projection of a Weyl
point, thus providing a clean signature.